Starter quiz
- Three beads are red and seven beads are green. What percentage of the beads are green?
- '70%' ✓
- Balance the following chemical equation: CH₄ + O₂ → H₂O + CO₂
- CH₄ + 2O₂ → 2H₂O + CO₂ ✓
- 2CH₄ + O₂ → 2H₂O + CO₂
- 2CH₄ + O₂ → H₂O + 2CO₂
- CH₄ + 2O₂ → H₂O + CO₂
-
- Calculate the relative formula mass (Mr) of calcium carbonate, CaCO₃. Relative atomic masses (Ar): Ca = 40; C = 12; O = 16.
- '100' ✓
- Which of the following statements about the equation below are correct? 2Mg + O₂ → 2MgO
- The ratio of magnesium to magnesium oxide is 1 : 1 ✓
- The ratio of magnesium oxide to oxygen is 1 : 2
- The ratio of oxygen to magnesium is 1 : 2 ✓
- The ratio of oxygen to magnesium oxide is 2 : 1
-
- When magnesium reacts with oxygen in the air to form magnesium oxide, magnesium is the limiting factor. There is more oxygen present than is needed, so we say that the oxygen is in ______.
- 'excess' ✓
- Calculate the mass of 0.5 mol of magnesium. Give your answer to 2 s.f. Relative atomic mass (Ar) of Mg = 24.
- '12 g' ✓
Exit quiz
- Match the following terms to the correct definitions.
- theoretical yield⇔Maximum calculated mass of product expected to form in a reaction. ✓
- actual yield⇔Amount of product obtained from a chemical reaction. ✓
- atom economy⇔Percentage of reactants converted into useful products. ✓
- During an experiment, a pupil produces 3.25 g of magnesium oxide. The theoretical yield of magnesium oxide is 4.00 g. Calculate the percentage yield. Give your answer to 3 s.f.
- '81.3%' ✓
- Why might the percentage yield of a reaction not be 100%?
- The reaction is reversible. ✓
- The reaction goes to completion.
- Product may be lost during transfer. ✓
- All the reactants are used in the reaction.
-
- Calculate the theoretical yield of magnesium oxide when 2.40 g of magnesium reacts with excess oxygen. Give your answer to 3 s.f. 2Mg + O₂ → 2MgO Relative atomic masses (Ar): Mg = 24; O = 16.
- '4.00 g' ✓
- Which of the following statements about atom economy are correct?
- It is a measure of the efficiency of how a reaction uses its reactants. ✓
- It shows how actual yield compares to theoretical yield.
- It assumes the reaction is not complete.
- It assumes no side reactions occur. ✓
-
- Calculate the percentage atom economy of the reaction below, when hydrogen is the useful product. Give your answer to 3 s.f. 2H₂O → O₂ + 2H₂ Relative atomic masses (Ar): H = 1; O = 16.
- '11.1%' ✓
Worksheet
Presentation
Video
Lesson Details
Key learning points
- In an ideal chemical reaction, all atoms in the reactants would end up in useful products, with no waste.
- The theoretical yield of a chemical reaction can be calculated from the reaction equation alone.
- The percentage yield shows how actual yield compares with theoretical yield.
- Atom economy is a measure of the efficiency with which a reaction uses its reactants.
- Many factors will be considered when choosing a reaction pathway.
Common misconception
Pupils often confuse percentage yield with atom economy, thinking both measure the same efficiency aspect.
Clarify that percentage yield measures actual vs. theoretical product amount, while atom economy measures reactants' mass converted to desired product.
Keywords
Theoretical yield - is the maximum calculated mass of product expected to form from a given mass of reactants
Actual yield - is the actual amount of product obtained from a chemical reaction
Percentage yield - is the actual yield (mass of product obtained) divided by the theoretical yield (maximum mass of product possible), as a percentage
Atom economy - is the percentage, by relative mass, of reactants that are converted into useful products
Useful product - is a chemical product that is desired or valuable